This invention relates to temperature-compensated crystal oscillators (TCXO) and more particularly to temperature-compensation voltage generator circuits for use in such oscillators.
Conventionally, a temperature-compensated crystal oscillator of the type concerned in this invention is composed of a voltage-controlled crystal oscillation circuit including a variable-capacitance diode or varactor and a quartz crystal unit. A temperature-compensation-voltage generator circuit includes at least one thermistor and a plurality of resistors. The oscillation frequency of such voltage-controlled crystal oscillation circuit generally has a certain temperature characteristic which is also variable with the capacitance of the variable-capacitance diode. Such an oscillation frequency can, therefore, be held constant by adjusting the varactor capacitance in relation to the frequency-temperature characteristic of the oscillation circuit. In other words, it suffices for the compensation purpose to supply an appropriate control voltage from the temperature-compensation-voltage generator circuit to the variable-capacitance diode.
In general, however, the range of the frequency variation of voltage-controlled crystal oscillators is not very wide. Therefore, it is desirable to begin by selecting a quartz crystal unit having an oscillation-frequency vs. temperature characteristics which lies within a certain definite range. Then, the temperature characteristic of the compensating control voltage is selected so that the oscillation frequency is held constant by varying the ambient temperature of the oscillator. Next, a voltage generator circuit is designed to produce a compensation voltage having approximately the selected temperature characteristic.
For any of other oscillators, such a voltage generator circuit must be designed separately by the same procedure that is described above. In other words, such a voltage generator circuit must be designed for an exclusive use with each of the oscillators. A result is that mass production cannot be applied thereto. Inevitably there is an extraordinarily high labor cost for their manufacture and adjustment. The design procedure described above is, therefore, economically undesirable particularly with crystal oscillators required to have a frequency stability (.DELTA.f/f) in the order of .+-.5 ppm within a range of 31 30.degree. C. to +70.degree. C. For a description of such oscillators, reference may be had to the paper, (1) "A New Approach to a High Stability Temperature Compensated Crystal Oscillator" by S. Schodowski, Proceedings of the 24th Annual Symposium on Frequency Control, pp. 200-208, 1970.
Another known form of crystal oscillator, is disclosed in the paper, (2) "A Digitally Compensated TCXO" by G. E. Buroker et al., Proceedings of the 27th Annual Symposium on Frequency Control, pp. 191-198, 1972. This oscillator includes a temperature-compensation-voltage generator circuit consisting of a temperature-to-frequency converter, a counter, a digital memory, a D/A converter and an analog memory. This circuit, however, is very complicated, of high cost, and is not practical for general-purpose crystal oscillators.
Further references are cited below for a better understanding of the present invention
(3) Shuzo Fujii et al, "Improvement of Frequency Stability for TCXO," NEC Research and Development, No. 43, pp. 75-80, October, 1976.
(4) Ishihara, "Temperature-Compensated Crystal Oscillator," Nippon Dempa Kogyo K.K. Technical Report, pp. 100-109, 1974.
(5) Japanese patent publication No. 47-34091 (published Aug. 29, 1972).